This invention pertains generally to directive antennas for radio frequency energy and particularly to wide-band directive antennas for radio frequency energy.
It is known in the art that an array of antenna elements may be fed through a parallel plate lens, i.e. a "microwave" lens, and a plurality of transmission lines in such a manner than one, or more, beams of radio frequency energy are formed. With proper design, such an assembly may be operative over a wide band of frequencies, say an octave band. Because the principle of reciprocity applies, such an antenna assembly is also adapted to receive radio frequency energy within the same frequency band from one, or more, directions.
In one known antenna assembly of the type just mentioned, a design defining a linear array of antenna elements, transmission lines, microwave lens and a plurality of feedports are formed on a common dielectric substrate using printed circuit techniques. After the so printed dielectric substrate is assembled in operative relationship with one or two ground planes (depending upon whether a microstrip or a stipline assembly is desired), constrained paths in the dielectric substrate are defined for radio frequency energy within a relatively wide frequency band. The dimensions of, and spacing between, the various parts of the printed design determine the characteristics of the completed antenna assembly. In particular, with a plurality of feedports along a focal arc, the printed design is so arranged that the electrical lengths of the paths between each feedport and the antenna elements are systematically controlled. When all of the feedports are energized, the phase shifts experienced by radio frequency energy passing from each feedport to the antenna elements are such that a plurality of simultaneously existing beams of radio frequency energy is formed, each pointing in a different direction. The same antenna assembly may be operated to form a single one of the beams by simply energizing a single one of the feedports. While such an antenna is adapted to operation over a wide band of frequencies, experience has proven that the beamwidth of its radiated beam, or beams, varies inversely with frequency.
While a variation in beamwidth due to a change in operating frequency may be tolerated in many applications, cases exist where such a variation seriously affects proper performance. For example, if (when the antenna assembly is to produce a plurality of simultaneously existing beams) it is desired to maintain the power level at the crossover point between adjacent beams, any variation in beamwidth due to a change in operating frequency obviously should be avoided. Similarly, if (when the antenna assembly is to produce a single beam) it is desired to reduce clutter when a beam is pointed so as to graze an extended area, as the sea or a land mass, it is also obvious that any variation in beamwidth due to a change in operating frequency should be avoided.
One technique described in pending patent application Ser. NO. 442,704 filed 2/15/74, inventor W. B. Hatch, assigned to the assignee of the present invention, provides a "constant beamwidth antenna" wherein an array of antenna elements is disposed to form at least one directive beam, by providing attenuator means in circuit with selected ones of the antenna elements in such an array, individual ones of such attenuator means having frequency response characteristics such that the amplitude taper of the electromagnetic energy across the array is varied as the operating frequency is changed. The attenuator means comprises a number of low pass filters having different cutoff frequencies within a band of operating frequencies, such filters being disposed in circuit with selected antenna elements so that, as operating frequency is increased, the number of energized antenna elements is decreased to maintain the size of the effective aperture of the array at a substantially constant size, measured in wavelengths. While such an antenna assembly has been found adequate in many applications, the antenna assembly of the present invention is an improvement thereon because the contemplated assembly may be constructed more simply and inexpensively than such known antenna assembly.